In today's era of handheld devices, in order to attract the attention of consumers, the design of today's mobile phone has become diversified and personalized. Especially, the design at the back of the mobile phone, apart from an eye-catching logo, usually consumers are allowed to add in the blank space any variety of decorations to suit his/her personal preference, such as 3D stickers, etc.
Referring to FIG. 1, a conventional protective case assembly includes a casing body 1 for covering a mobile phone (not shown), and a back plate 2 detachably connected to the casing body 1. However, the back plate 2 blocks the original or personal special design at the back of the mobile phone. Thus, in order to protect the mobile phone, the special design at the back of the mobile phone must be sacrificed. Further, the back plate 2 may also hinder the heat dissipation and wireless charging of the mobile phone. Hence, a user must detach the back plate 2 from the casing body 1 to expose the design at the back of the mobile phone, to increase wireless charging efficiency, and to accelerate the heat dissipating rate of the mobile phone. However, this will result in the exposure of the engaging structure of the casing body 1 and the back plate 2, so that not only the overall outer appearance of the mobile phone is affected, dirt will also be easily accumulated in the mobile phone.